In recent years, development of high power modules such as high-power high-efficiency inverters has been underway with an increase in performance of industrial equipment such as robots and motors. In proportion to outputs of high power modules, heat generated from semiconductor elements is steadily increasing. To dissipate (radiate) the heat efficiently, various methods are adopted for high power modules. Recently, use has been made of a ceramic circuit board including metal plates attached and bonded on both sides of a ceramic substrate.
As the ceramic circuit board, the circuit boards described in WO 2007/105361 (Patent Document 1), Japanese Patent Laid-Open No. 2010-118682 (Patent Document 2) are developed. According to Patent Document 1, an improved TCT property (thermal cycling resistance property; thermal cycle test property) is given by performing control of a thickness ratio between a metal plate on a front side (first metal plate) and a metal plate on a rear side (second metal plate) to 50% or larger and 200% or smaller.
According to Patent Document 2, a ceramic circuit board that warps so as to be convex toward a metal plate on a front side (on a metal circuit plate side) is developed. According to Patent Document 2, by adopting such a structure, an improved solder flow property is given. By adopting the structures of the ceramic circuit boards as described in Patent Document 1 and Patent Document 2, a TCT property and a solder flow property are improved.
In accordance with an increase in power of semiconductor elements, a further improvement of a heat dissipation property has been demanded. In a semiconductor module, a semiconductor element is mounted on a ceramic circuit board through a solder layer interposed therebetween. In such a module structure, a heat transfer path is as follows: semiconductor element solder layer metal plate (metal plate on the front side) ceramic substrate metal plate (metal plate on the rear side). One of indexes indicating a heat dissipation property of a ceramic circuit board is thermal resistance. A lower thermal resistance indicates a better heat dissipation property.A thermal resistance (Rth) is determined as Rth=H/(k×A).Here, a symbol H represents heat transfer path, k represents thermal conductivity, and A represents heat dissipation area. As is clear from the above calculation expression, decreasing the thermal resistance (Rth) involves shortening the heat transfer path (H), increasing the thermal conductivity (k), and increasing the heat dissipation area (A).
Moreover, improvement of a TCT (thermal cycle test) property is also demanded for ceramic circuit boards. According to Japanese Patent No. 3797905 (Patent Document 3), a silicon nitride substrate having a three-point flexural strength of 500 MPa or higher is developed. Use of a silicon nitride substrate having a high flexural strength (bending strength) allows the improvement of the TCT property.
To cope with the increase in power mentioned before, further improvement of the heat dissipation property and the TCT property have been demanded. To improve the heat dissipation property by using a high-strength silicon nitride substrate, it is effective to increase the heat dissipation area (A). To shorten the heat transfer path (H), it is effective to reduce thicknesses of a metal circuit plate and a ceramic substrate.
However, an excessive reduction of the thickness of a metal circuit plate makes it difficult to flow large current. An excessive reduction of the thickness of a ceramic substrate causes concern about an insulation property. As to the thermal conductivity (k), an aluminum nitride substrate having a thermal conductivity to a degree of 250 W/m·K is developed. However, such the aluminum nitride substrate has a strength of only about 250 MPa and thus is poor in TCT property.
Examples of effective methods to increase the heat dissipation area (A) include a method to enlarge a metal plate to be attached to a ceramic substrate, and a method to attach a leadframe, a heat sink, and the like.